1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus, and more particularly to a discharge lamp lighting apparatus to light a plurality of discharge lamps for use as a backlight in a liquid crystal display (LCD) apparatus.
2. Description of the Related Art
An LCD apparatus, which is a flat panel display apparatus, is used in various applications. Since a liquid crystal in the LCD apparatus does not emit light by itself, a lighting device is required in order to achieve a good display. A backlight device to light a liquid crystal panel from behind is among such lighting devices. In the backlight device, a cold cathode lamp is mainly used as a discharge lamp, and a discharge lamp lighting apparatus including an inverter to drive the cold cathode lamp is provided.
Recently, the LCD apparatus is becoming larger and larger for use in, for example, a large-screen TV, and therefore a number of discharge lamps are used in a backlight device in order to achieve sufficient screen brightness for the LCD apparatus. Some backlight devices are provided with such a function as to variably control the brightness (luminance) of the discharge lamps depending on the environments. What is called “burst mode dimming method” is one of the brightness control methods. The burst mode dimming method operates such that driving power supply voltage is intermittently outputted so as to provide discharge lamps with on-periods and off-periods, and the ratio between the on-periods and off-periods which are defined by intermittent operation of high frequency current flowing through the discharge lamps is varied thereby controlling the time-average brightness.
In the burst mode dimming method, however, when a plurality of discharge lamps are intermittently lighted on and off simply by a prescribed output waveform, currents flowing in all the discharge lamps are switched on and off concurrently, and if this operation is repeated, then a current ripple of a switching power supply to apply a voltage increases, and consequently load current of the switching power supply must be increased thus causing cost increase on the switching power supply.
In order to deal with such a current ripple issue, what is called “a multi-phase discharge lamp lighting apparatus” is proposed, in which output phases of control circuits to control on/off operations of the discharge lamps are shifted from one another thereby controlling a ripple ratio (refer to, for example, Japanese Patent Application Laid-Open No. 2002-15895).
FIG. 3 is a block diagram for a circuitry of a discharge lamp lighting apparatus disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2002-15895. The discharge lamp lighting apparatus shown in FIG. 3 is adapted to drive a plurality of cold cathode lamps 107, includes a timing signal generating circuit 103, and a plurality of dimming signal generating circuits 104 and switching circuits 105, which are provided respectively in a number equal to the number of resonant circuits 106 connected to respective cold cathode lamps 107, and operates such that the timing signal generating circuit 103 receives a PWM timing signal 101 and sequentially selects one dimming signal generating circuit 104 thereby sequentially turning on one cold cathode lamp 107 connected to the one dimming signal generating circuit 104 selected.
In the discharge lamp lighting apparatus shown in FIG. 3, a current ripple of a switching power supply can be suppressed by shifting on/off phases of the cold cathode lamps 107 from one another. However, since the dimming signal generating circuits 104 and the switching circuits 105 must be provided individually for each of the resonant circuits 106, a great number of control IC's and other components are required thus resulting in cost increase on the discharge lamp lighting apparatus.